Normalized degree of polarization as a function of wavelength for a two-layer system (top) and a three-layer system (bottom) in M 51. The plots show some representative model configurations from Shneider et al. (2014a). The observed degrees of polarization in region “A” are displayed with error bars. None of the model configurations can fit the data in the S-band (around ∼10 cm). All model profiles featured have been constructed from the set of parameters given in Table 5 (but using α = 1 for the halo): a regular field strength of 5 μG in the disk and in the halo, a disk turbulent field of 14 μG, and a halo turbulent field of 4 μG (see Table 6 for nomenclature and description of the model types listed in the legend).